Physics of Black Holes

I am a subscriber to newscasts from the Hubble Space Telescope project, mostly for the fabulous images that I get sent. I was browsing through some of their stuff today (link)and was looking at the image below, a new composite one of galaxy cluster MS0735.6+7421. The composite is derived from a combined optical (yellow), X-ray (blue) and radio (red) image. The X-ray image shows hot gas enveloping the cluster, with holes in the regions where there is radio activity. The radio activity is due to two jets emerging from a supermassive black hole at the centre of the cluster (bright spot at centre). It is estimated that the black hole has a billion solar masses. This galaxy cluster is 2.6 billion light years from Earth, in the constellation Camelopardalis. The optical data was obtained with Hubble's ACS instrument, the X-ray data from the Chandra X-ray Observatory's ACIS instrument, and the radio data from the Very Large Array.

The NASA text accompanying the image, is:

Quote

The Hubble image shows dozens of galaxies bound together by gravity. In Jan. 2005, astronomers reported that a supermassive black hole, lurking in the central bright galaxy, generated the most powerful outburst seen in the universe. The VLA radio image shows jets of high energy particles (in red) streaming from the black hole. These jets pushed the X-ray emitting hot gas (shown in blue in the Chandra image) aside to create two giant cavities in the gas. The cavities are evidence for the massive eruption. The X-ray and radio images show the enormous appetite of large black holes and the profound impact they have on their surroundings.

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My bold text is the part central to this thread.

So. So my understanding until just now has been that black holes (and especially the big whoppers) exert such a strong gravitational field that NOTHING, not even light can escape from them. That's why they're called black holes. I've just looked them up here and there's nothing obviously wrong with my understanding.

If there are any real (or humorous, non-qualified) physicists on MR who could explain this to me, I would be grateful....

This might clarify it:
Though Hawking has not yet revealed the detailed maths behind his finding, sketchy details have emerged from a seminar Hawking gave at Cambridge. According to Cambridge colleague Gary Gibbons, an expert on the physics of black holes who was at the seminar, Hawking's black holes, unlike classic black holes, do not have a well-defined event horizon that hides everything within them from the outside world.

In essence, his new black holes now never quite become the kind that gobble up everything. Instead, they keep emitting radiation for a long time, and eventually open up to reveal the information within. "It's possible that what he presented in the seminar is a solution," says Gibbons. "But I think you have to say the jury is still out."

My understanding is that the reson light can't escape a black hole is because space bends because of large masses, and since a black hole is an extremely dense object the gravity is so strong that it bends light all the way back to the hole.

It's a hard idea to wrap your head around, and doesn't make sense to us but i think its really interesting.

There was some sort of theory that the universe might be bent by the mass of itself so that if you traveled forever in one direction you would end up back at the point where you started, now that is interesting.

So the (very) basic idea of Hawking's hypothesis, as I understand it, is that particles cannot escape from inside the event horizon of a black hole in the traditional / relativistic sense, but they do have a probability of tunneling out in the quantum mechanical sense (à la the Heisenberg Uncertainty Principle, very loosely). This is one of those situations where quantum mechanics and relativistic gravitational theories need to be reconciled.

However, I'm not so sure at all that Hawking radiation is what is being implicated here. A separate issue is that, even though matter cannot escape from the black hole once inside the event horizon, according to relativity, nothing prevents a hyperbolic approach course that causes the matter to be drawn initially closer to the event horizon and then veer back away from it before reaching it. In other words, the matter is not really coming from "inside" the black hole, but is rather being accreted by it and then ejected in plumes because of the hyperbolic trajectory it's in.

My fragile and likely wrong understanding is that these jets do not emerge from the black hole proper, but instead from interactions within the vortex of material streaming into it; I believe that the rotation of the hole creates this effect and that, were one to have a non-rotating black hole, such plumes would not exist.

Black holes suck material toward them, but some of it gets spit out rather than swallowed. Many black holes eject jets that move away from the accretion disk at nearly the speed of light. These jets have been observed most spectacularly from the centers of nearby galaxies (for example, M87) but also appear in microquasars - in quick, enormously energetic spurts and sputters, as if someone had taken a video of a quasar jet and pressed the fast-forward button.

The processes by which these jets are formed are not well understood, but seem to require magnetic fields - whose presence causes instabilities in the accretion disk that allow material to fling upwards - as well as rapidly rotating black holes, which can feed some of their energy to the magnetic field and to the jet material itself.

~~ SNIP ~~ According to Cambridge colleague Gary Gibbons, an expert on the physics of black holes who was at the seminar, Hawking's black holes, unlike classic black holes, do not have a well-defined event horizon that hides everything within them from the outside world. ~~ SNIP~~

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That New Scientist article was very helpful indeed, although I get lost in the detail so quickly whenever I read anything that Hawkins writes. A load more reading tonight, then. Thanks a lot for the reference.

the ol ball on a piece of fabric to simulate gravity's effect on the curvature of space time.

we've all seen it, and its a wonderful model.

problem is, its only part of the real picture.

what you have to realize is that the deformation of space time, the dent from the ball, is actually occuring on every side of the ball. Not just the bottom, as there is no bottom in space.

understanding that space time is curved by gravity around a sphere, is hard to visualize because you cant, but if you take the ball and fabric and imagine the dent to be happening at every point around the sphere, it might make a little more sense.

black holes, though large spinning discs around a central point, do pull from the entire 3D sphere of space, but I think the movement of the disc causes it to bulge and have places where the gravity isnt strong enough to pull the ejected radiation back in.

That New Scientist article was very helpful indeed, although I get lost in the detail so quickly whenever I read anything that Hawkins writes. A load more reading tonight, then. Thanks a lot for the reference.

For Hawking radiation (which shows how something - sort of - can escape a black hole), you can think of it this way...

Imagine the event horizon of a black hole, where everything on one side gets sucked in, and everything on the other side makes it out. Quantum theory says that particle/antiparticle pairs are fluctuating in and out of existence all the time, so when they appear right at the event horizon, one half can escape, while the other half gets dragged in by gravity.

The particles that escape give the appearance from outside that the black hole is radiating energy, and the particles that get sucked in have a negative energy impact and give the appearance that the black hole is losing mass.

Although that's not what's going on here - I believe it's gas/matter that collects/orbits around the black hole (pulled in by the extreme gravity), which eventually builds up and becomes unstable. It eventually gets ripped apart once it hits a critical mass - some of it gets sucked into the black hole and the rest gets spit out in a violent jet.

The one thing that doesn't get trapped by black holes is gravity... which in turn can be considered a form of energy.

What Hawking proposed back in the mid 70's as I recall (and now known as Hawking Radiation) is a marriage of two areas of physics (quantum mechanics and general relativity) to provide a solution for one of the oldest laws in physics... conservation of energy.

As most people who have heard of black holes know, nothing can escape them, not even light. But this concept troubled physicists because this idea lead to loss of energy. The relativistic math of black holes was very straight forward and became evident with the Schartzschild Solution*, but that left a lot of other questions unanswered. Infact, the name Black Hole was originally a term of ridicule towards people that believed in these objects.

What was put forward by Hawking was based on virtual particles from quantum theory in which everywhere in the universe particle-pairs (a particle and an antiparticle) pop into and out of existence**. Hawking realized that at the event horizon*** of a black hole the gravitational tidal forces could be so great as to separate these virtual particles from each other. Most of the time, both particles get swallowed up by the black hole, but every so often half of a virtual pair might escape. The loss of that particle translates into a loss of energy for the black hole. The energy from the black hole was transmitted by gravity to the virtual pair of particles and was lost when part of the pair escaped. The particle being given off from the black hole is Hawking Radiation.

Hawking theorized that if this happened enough, the black hole would eventually lose so much energy that it would evaporate completely.

Please note that the particles in Hawking Radiation do not have a causal path**** back to the black hole itself. Because of this, no information from a black hole escapes.

I think that covers most of the information requested without having to get into the mathematics of all this (which is really great, and how I fell in love with mathematics, and specifically differential geometry and differential topology to begin with).

* The Schartzschild Solution is an application of general relativity centered around a spherical mass... which as I recall was one of the first applications of mathematics developed by Levi-Civitia to provide the ability to apply general relativity at all. Before Einstein's theory of gravity was released in 1916, the area of differential geometry in mathematics had changed very little since the mid 19th century, mainly with the work of Riemann. General relativity revitalized differential geometry in many ways.

** Virtual pairs are best thought of as a single particle traveling in a circular path in space-time, which while moving forward in time it appears as a particle, and which while moving backwards in time it appears as an antiparticle.

*** The event horizon of a black hole is a sphere with a radius equivalent to twice the mass of the black hole. What was noticed in the Schartzschild Solution was that if the radius of a massive body became smaller than this Schartzschild Radius, the body would continue to collapse without end... this part of the solution produces a singularity in local space-time which is outside what the mathematics can deal with (which is why we don't know much about what happens in a black hole).

**** An important aspect of space-time singularities is that they are isolated from the rest of the universe. This means that there are no causal paths back inside of an event horizon. This is generally put forward as there can be no naked singularities (where the singularity is the black hole itself). This idea was altered slightly with Big Bang Theory because the Big Bang is a singularity and everything has a causal path back to it.​

Suggested Reading and References:

Gravitation by Charles Misner, Kip Thorne and John Wheeler

Large Scale Structure of Space-Time by Stephen Hawking and GFR Ellis

Gravitational Curvature by Theodore Frankel

Techniques of Differential Topology in Relativity by Roger Penrose

The Geometry of Physics by Theodore Frankel

For full disclosure, Professor Frankel was a professor at UCSD while I was attending. I have met (but do not know personally) both Professor Thorne and Professor Penrose... just in case someone thinks any of these suggestions are to push book sales.

Thank you everyone for your input into my query. You have all helped to make things a load clearer to me and the references/links that you provided have all been either read or filed for future reading.

In particular, I would thank RacerX for his detailed, erudite and interesting response.

I was thinking about this the other day. And it hit me that once mass collapses to event horizon that time stops at the limit and all the matter of black hole at that limit is frozen. The rest of the mass inside is at a slightly lower gravity but can still collapse some and it does until it reaches the smaller limit and so on. So a black hole would actually be nested shells.

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